Current regulation



Aug. 12, 1952 L. A. MEACHAM 7,03

' CURRENT REGULATION Filed April 17, 1948 2 SHEETS-SHEET 2 /5 KJ me P2OUTPUT 1 e "arr" I t; MON 2 i b INVENTOR L. A. MEA CHAM ATTORNEYPatented Aug. 12, 1952 CURRENT REGULATION Larned A. Meacham, NewProvidence, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationApril 17, 1948, Serial No. 21,651

12 Claims. (Cl. 32322) This invention relates to current regulation,

The principle object of the invention is to render the current flowingfrom a source through a load to a great extent independent of loadvariations. A subsidiary object is to switch such a current on and offunder the control of suitable pulses, thus making it a substantiallytwo-valued function of time, the selection of one or other of the valuesbeing fully controlled, both as to its existence and as to the instantat which it comes into existence, while the magnitude of the particularvalue selected remains independent of past selections and otherdisturbing influences.

The invention is of use wherever it is required to draw through anetwork a current whose magnitude is independent of voltage variationsacross the network, but which is controllable as to the instants of itsinception and termination. For example, in charge-measuring circuits ofcertain types, such a current of standard magnitude and controlledduration constitutes a measure of the charge accumulated on a condenseras a result of the current flow through it. The invention is equally ofuse in the generation of a sweep voltage Wave of truly saw-tooth formfor application to th deflecting elements of a beam tube and, generally,wherever a current is required to be precisely controlled as to thetimes of its inception and termination but otherwise independent of itsenvironment,

The stated objects, and others as Well, are attained by the provision,in series with a current source and a load in which the current is to beheld constant independent of variations in the load, ofa high resistorand two or more triodes connected for cathode feedback, the resistorconstituting the cathode feedback resistance of one triode, and thistriode and its cathode resistor together constituting the cathodefeedback resistance of a second triode. In accordance with thewell-known characteristic of the conventional cathode feedbackconnection which affords an effective plate circuit impedanceapproximately equal to the resistance of a cathode resistor increased bya factor 1+ where is the amplification factor of the tube, the effectiveincrease which is consequent upon the interdependent tubes of theinvention is multiplicative; that is, it is approximately (1+,u.) 2 fortwo tubes, (1+ 3 for three, and so on. This affords a marked improvementover the series arrangement of independent cathode feedback tubes,wherein the effects are merely additive, i. e., 2(l+ for two tubes, 3(1+for three tubes, and so on.

The foregoing arrangement is well adapted to be combined with abalancing circuit which, nominally, results in an infinite effectiveimpedance, but with which serious deviations from the ideal Incombination with the interdependent triode arrangement, the sensitivityof the balance circuit to such departures is greatly reduced.

Whether the circuit be employed alone or in this or other combinations,the current, which as explained is constant while on, may be freelyswitched off and on again by switching or gating pulses applied to it ata suitable point. When this application is made by way of a suitablyenergized gating tube, the switching instants, as well as the magnitudesof the switched currents, may be rendered substantially independent ofload variations.

The invention will be fully apprehended from the following detaileddescription of certain preferred embodiments thereof, taken inconnection with the appended drawings, in which:

Fig. 1 is a schematic circuit diagram illustrating the principles of oneaspect of the invention;

Fig. 2 is a schematic circuit diagram illustrating the principles ofanother aspect of the invention;

Fig. 3 is a schematic circuit diagram of a preferred embodiment of theinvention, combining the principles of Fig. 2 with those of Fig. 1; and,

Fig. 4 is a wave form diagram of assistance in the explanation ofcertain aspects of the invention.

Referring now to Fig. 1, a network of impedance Z, denoted by the blockI, is shown connected in series with a current source which may beprovided by batteries 2, 3 and with two triodes 4, 5 and a resistor Rx.The network I may, for example, include a condenser to which standardincrements of charge are to be applied by passing through it currents ofstandard magnitudes which endure for standard times. The anode of theupper triode 5 is connected to the lower terminal of the network I. Itscathode is connected to the anode of the lower triode 4. The cathode ofth latter is connected to one end of the resistor Rk. The other end ofthe resistor Rx is connected to the negative terminal of the batteries2, 3 Whose positive terminal is connected to the upper terminal of thenetwork I, thus completing the series circuit.

The control electrode or grid of the lower triode 4 is maintained at afixed potential, for example by connection to a tap 6 of the battery 3.Ourrent flowing through the lower tube 4 and the resistor Rk produces avoltage drop across the latter, thus raising the cathode to a potentialslightly higher than that of the grid, by the action known as cathodefeedback.

Disregarding, for the present, the action of the switching tubes 8, 9,that is, assuming them both to be cut off, if

i zcurrent through lower triode, ek =cathode potential of lower triode,ep =anode potential of lower triode,

e z rid potential of lower triode, i amplification :factor of lowertriode, R =plate resistance of lower triode,

where all potentials are measured with respect, to a common point, suchas the lower end of the" resistor Rs, then, from elementary circuit.considerations,

e e;,- +m e e,

l 1 1) (ill) de deg macho? ,7

But the variation of 'thefcathode potential with the tube .current issimply and therefore (2) becomes fi l 1 (4) This quantity, in turn, issimply the reciprocal of the apparent resistance R1 looking into theanode of the lower. tube; and thereto-re 1 l o 31 711 11 -+I '.L) k

Thus the uppertube too is connected for-cathode feedback, its feedbackresistor being furnished discharge path of the lower tube and theresistor Rk'together. Equations ofthe same form as the foregoing onesmay therefore be written for the er tube, in which:

i is :the current through it, ,e is its anode voltage,

"e1; is its cathode volt-age, c is its grid voltage,

,uz is its amplification factor. B is its plate resistance,

and R1, as givenby (5) above is its cathode feedback resistance. Thus,by analogy with (2),

R132+(1TIL2) dip Because the cathode the upper tube is connecteddirectly to the anode of the lower one,

6k =p dc de,, a; m

4 Substitution of (9) and l) into (7) gives infill f pf p +a i +u+m l+mm where R2 is now the apparent resistance of the two tubes and thecathode resistor acting in combination.

if Bk is of substantial magnitude as compared with the plateresistances, and if 1 and as are substantially greater than unity, (10)becomes, approximately Rz:(1+ 1)(l+ l2)Rk (11) If, in turn, the tubesare alike,

, swim V 'so'that' 1 Ra (-l-l-u) Rk 12') Thus the circuit arrangement-ofthe invention results ina multiplicative increase in the eflect producedby a single cathode feed'back. tube by itself. .It is, indeed, equal tothat which would be obtained by use of a "very large number 10findividually independent cathode feedback tubes in :series, and :isobtained without resorting to a current source of excessive voltagewhich would be required for-such an arrangement. ,Any varia- 'tions inthe impedance Z of the network l or in the voltage :across sit, such asan inductive voltage due to suddent application or interruption of theregulated current, or :the voltage :due ".to an accumulation of chargeon :a :condenser, ;are prevented from causing changes :in the current'throughxthis network by the much higher im- -p edance of theintercoupled cathode ieedhack triodes,

The current through the circuit which, it exists, is substantiallyconstant as described above, may be switc'hedon and oil bysgatingpulsesderived from sources not shown .and applied to the :cathode of :theflower tube, :the cathode of the upper tube, or elsewhere. By -way ofexample, two sequences of :gating'pulses are shown in the figure asbeing applied by way of gating :tubes 8, 9, :the first sequence being'an 'irregularsequence .of relatively long pulses and the second aregular sequence of short ones. With proper selection of "the: gridpotentials of the gating tubes with respect to those of the-seriestriodes, either of thegating tubes robs the network I of all .currentuntil the simultaneous :arrival of pulses at both gating points drivingthe gating tubes .below cut-oil. Thereupon current 'flOWs throughthmnetwork 1 and the series triodes during their 'joint :presence andceases when either :of them terminates. With a different selection :ofthe tapping points 6-, l, and opposite polarity of the pulses, thegating tubes maybe arranged to turn the circuit off insteadotfion. Inthis case, the-current is interrupted when apulse occursat either ofthegating points.

The principle 'of operationhere is similar to that described inapplication of L. A. Meacham, Serial No. 772,913, filed September '9,1947, now Patent 2,537,843, issued January 9, I951.

The Voltage at an output point, such as a terminal [0 of the impedancenetw0rk,.is .a'precise measure of the charge which has been accumulatedon :a condenser contained therein by the flowof' constant current inresponse to the gating pulses. l

The arrangement may obviously be extended to three -or more stages, itbeing only necessary to connect a desired number of triodes inseries,

theanode of each to the cathode of'the next, to provide a highresistance cathode resistor for the first in the conventional fashion,and to connect the control grids of all of them to points of fixedpotential of appropriate value.

Fig. 2 shows a circuit arrangement in which the problem of maintainingconstancy of current through the load impedance network I is approachedin a difierent manner. The impedance network i is connected in serieswith a potential source such as a battery [2, a triode l3 and a highresistor Rk connected from the cathode of the triode to the negativeterminal of the battery l2. The control grid of the triode is suppliedwith a fixed potential as by connection to a tap M of the battery I2. Asabove explained, these connections provide inverse feedback by way ofthe cathode resistor Rk so that the efiective impedance in series withthe network I is given by Equation 5. A second cathode feedback triodel5 has its anode connected directly to a suitable fixed positivepotential point of the battery l2 and its cathode resistor R0 from itscathode to the negative battery terminal. Output voltage may be takenfrom the cathode of this triode l5,

which is thus connected as a cathode follower,

while input switching pulses may beapplied by way of a buffer amplifier16 to the cathode of the first triode, Or otherwise, as desired. Aresistor Rb is connected from the cathode of the first triode l3 to thatof the second. With this arrangement and proper choice of the value ofthe resistor Rb, it is possible to reduce the value of to zero, thusplacing an effectively infinite variational impedance in series with thenetwork I. This will be evident from the following:

It is a consequence of the behavior of the cathode follower tube 15that, when the voltage c of the left-hand triode I3 is applied to itsgrid, its cathode voltage will be where m is a constant which may bemade to have a value only slightly smaller than 1 by suitable choice ofthe tube and the resistor R0. From elementary circuit analysisconsiderations,

Substituting this expression into Equation 2, and

collecting terms, gives The numerator of this expression, andconsequently the expression as a whole, vanishes when the resistor Rb ischosen to satisfy the relation Here, for a qualitative explanation, thefundamental relation of the triode l3, assuming constant plate current,namely may be taken as a starting point. If i is aremain constant, anyincrease in e must be accompanied by a smaller increase in ck, such thatEquation 18 is satisfied. But, if Rx is finite, the increase in ek, mustbe accompanied by a small increase in current through Rk. Thisadditional current must not be supplied by way of the triode [3, or theinitial assumption will be violated. Therefore, the proper amount ofcurrent is supplied from the cathode follower l5 to Rt: through theadded high resistor Rb. Efiectively, the oathode impedance of tube 13 isthus made to behave as though it were infinite.

Since #1 and (to a lesser extent) m depend upon the tubes themselves,the optimum value of Rh may change from time to time; but comparison of(16) with (5) shows that even a moderate degree of balance affordsimprovement over the results obtainable by use of a single cathodefeedback tube as a current regulator for the impedance network.

For example, with 1238, 12127000, Rk 30,()00, and 111:0.95, it turns outthat for true balance Rb:1.082 megohms. If Rb actually misses this valueby as much as i10%,

all

has a value in the range 10.09 micromho, corresponding to a seriesvariational resistance of 11 megohms.

Fig. 3 'is a schematic circuit diagram of a system in which balance isused to minimize residual variations not only in the magnitude of thecurrent, in accordance with the principles of Fig. 2, but also in theinstants at which the current is switched on or off. As in Fig. 1, animpedance network through which the current is to be held constant isshown connected in series with a source [2, two triodes, 4, 5, and ahigh resistor Rx, the anode of the lower triode 4 being connected to thecathode of the upper one 5, and the cathode of the lower one 4 to theresistor Rk. As before, the other end of the resistor is connected tothe negative terminal of the source l2 of which a suitable positivepoint is connected to the impedance network I. The control grids of thetwo triodes 4, 5 are supplied with fixed potential as by connection tosuitable taps of the source. The output is taken from the anode of theupper triode 5 and is applied to the control grid of a buffer amplifiertube [5 whose anode is supplied from the positive terminal of thesource. This tube may appropriately be connected as a cathode follower,in order to make the output voltage which appears across the impedancenetwork I available to succeeding circuits at a low impedance level.Thus a resistance RC is connected from the cathode of the follower tube15 to the negative terminal of the source l2. The circuit as thus fardescribed operates like that of Fig. 1, to produce a very high effectiveimpedance in series with the impedance network and the source. Inaddition, a resistor Rb interconnects the upper ends of the tworesistors Rx and Be, as in Fig. 2.

Switching or gating pulses may be applied to the cathode of either oneof the series triodes 4, 5, as suggested in connection with Fig. 1, orto both of them together as shown in Fig. 1. In the present example,they are applied, by way of a buffer tube Hi, to the cathode of theupper triode 5. This butler tube 16 is shown as being supplied withanode potential either from a fixed potential point of the source l2with the switch in position A or, with the switch 'in position B, from:in .Fig. 2. 'Thus Equations 14 and 15 apply in terms; that is, with ereplacing e .But, from Equations 10 and 3 By choosing Rb to have thevalue given by 20 which evidently dependson ,ui, #2, m and Rk, the

effective admittance of the circuit as given by 19 is reduced to zero..Itisfor this reason that the resistance Rb may be termed a balancingresistor. As a practical matter,.however, this per- .iect balance cannotbe maintained, because of deviations or fluctuations vin the values of.the parameters on which it depends.

of thehigh value of the denominator of 19, due

But, because in turn to the action of the series cathode feedback tubes,minor departures from perfect bal- .ance still leave a very low valuefor the admittance of the circuit. In one example, with R17 :Rp :7,000ohms ,ur:uz:38 Rk:10,000 ohms 'm:0 .05

.the value of Rt for balance is 14.44 megohms.

For a ten per cent error in the value of this resistor, the admittanceat .de,

as given by Equation 19 .lies in the range of .i0.0065 micromho,corresponding to an efiective resistance R2 for the circuit as 'a wholeof 150 megohms. This evidently represents a marked improvement over thestability of the simpler balancing arrangement of Fig. 2.

In .the foregoing analysis reactive currents suchas might be drawn byinterelectrode capacitances have not been taken into account. The

balance of Figs. 2 and 3 can be extended to these as well, thus makingthe reactive component of a dc,

extremely small, as well as the conductive component. For example, inFig. 3 ,Cp, Cr, and Cr,

represent the principal interelectrode capacitances involved with theseries cathode feedback triodes.

'01) can be :lumped with the impedance network. Cr can be balanced by acondenser Ct on condition that the frequencies of interest do not extendabove the range in which the transmission .m through the follower tubeis substantially a :real number .so that er and 772617 are approximately.in phase. To determine the required magnitude of the balancingcondenser, the currents flowing through these two capacitances may beequated. Thus When Rb is adjusted .for conductive balance in accordancewithEquation 20,

and .in this case Equation .18 may be applied. This may be written, forthe upper series triode,

Similarly, 'Ck may be balanced by :a :small condenser Cb whose magnitudemay be determined by equating currents as above. Thus d C =C (m 1 FromEquation :10

de [R (l +P-2 v p+ t #2) no and therefore the value of Cb for balanceis, from (24) The circuit of Fig. 3 as thus far described results in acurrent of a high degree :of constancy through-the impedance network Iand the series triodes 4, 5 when the circuit is switched on. As in Fig.2, the switching may .be carried out by the application of pulses to abuifer tube It whose cathode is directly connected to the cathode of theupper series triode. When the control grid of this bufier tube issufficiently positive, all of the current through the lower seriestriode flows through the bufier tube so that the current through theupper series triode 5 and the impedance network is cutoff. A typicalcontrol pulse e as applied to the control grid of the buffer tube isshown in Fig. 4. Assume that the steady bias of this grid is so chosenthat the transfer of anode current from the buffer 16 to the upperseries triode 5 is just half completed when the pulse is at potentialBe, that is 68. represents the :grid potential of the'bufier tube '16 atwhich half the regulated current flows through it and half through theupper series triode 5. This is merely a convenient way of denoting thepoint at which the switching nominally takes effect, and gives acorresponding on time ii.

If the upper series triode 5 and the bufier-tube are alike incharacteristics and have the same anode potential, the voltage es willbe equal to the potential of the grid f the upper series triode 5,

regardless of the value of the common anode potential. But suppose theanode potential of the buffer tube It is fixed, for example, by throwingthe switch S1 to position A. When the anode potential e of the upperseries triode 5 is reduced below this fixed potential, for example inthe course of the addition of successive incremental charges to thecondenser of the impedance network I, the normal current regulation bythe action of the series triode arrangement during the on conditiondrives the cathode of the upper series triode 5 to a lower potential.Accordingly the transfer potential of the switching action is reducedfrom the value 6a to another value such as ch. Inasmuch as the leadingand trailing edges of the switching pulse e are not infinitely steep;the duration of the on time is diminished from its proper value ii to adifferent and incorrect value 152. Such a variation of on time with egis objectionable in many cases. For example, in any circuit in which acurrent which endures for a pre-assigned time is used as a measure of acharge, constancy of the time during which the current flows is of equalimportance with constancy of the current.

This effect is overcome with the switch in position B in which the anodepotential of the buffer tube is supplied, not from a fixed point of thesource l2, but from the cathode of the output tube l5 which, as abovestated, is connected as a cathode follower. It is characteristic of thecathode follower circuit that its cathode follows its control grid inpotential, differing therefrom only by a small margin. Inasmuch as thegrid potential of the output tube is identical with the anode potentialof the upper series triode 5, it results from this connection that theanode of the upper series triode 5 and the anode of the buffer tube [5undergo substantially identical potential variations. Hence the transferpoint in Fig. 1 remains at voltage ea regardless of the value of 3Various departures in detail from the circuit arrangements describedabove maybe made without departing from the invention whose scope isdefined in the appended claims.

What is claimed is:

l. The serie combination which comprises a current source, a load inwhich the current of the source is to be held substantially constantindependent of variations in the load, and a constant current path whichcomprises a first discharge device having an anode, a cathode and acontrol electrode, a second discharge device having an anode, a cathodeand a control electrode, the anode of the first device being connectedto the cathode of the second device, a resistor having one end connectedto the cathode of the first device, the control electrodes of bothdevices being maintained at fixed potentials with respect to the otherend of said resistor, means comprising a switching tube for deprivingsaid load of said constant current, a cathode follower stage comprisinganother discharge device and a high cathode resistor, a connection fromthe anode of the second series device to the grid of said follower, anda connection from the cathode of said follower for supplying anodepotential to said switching tube.

2. In combination with a current source and a network, apparatu forholding constant the current flowing from said source through saidnetwork which comprises a first discharge device connected for cathodefeedback with its discharge path in series with the network, a seconddischarge device connected for cathode feedback with its discharge pathin shunt with the series combination of the network and the firstdevice, a connection for applying the anode voltage of the first deviceas a signal to a control electrode of the second device, and a balancingimpedance element connecting the cathode of the first device to thecathode of the second device.

3. In combination with a current source and a network, apparatus forholding constant the current flowing from said source through saidnetwork which comprises a first discharge device connected for cathodefeedback with its discharge path in series with the network, a seconddischarge device connected for cathode feedback with its discharge pathin shunt with the series combination of the network and the firstdevice, a connection for applying the anode voltage of the first deviceas a signal to a control electrode of the second device, and a balancingimpedance element connecting the cathode of the first device to thecathode of the second device, said impedance element having a value suchas to reduce the variational admittance of the discharge path of thefirst device to zero.

4. In combination with a current source and a network, apparatus forholding constant the current flowing from said source through saidnetwork Which comprises a first discharge device connected for cathodefeedback with its discharge path in series with the network, a seconddischarge device connected for cathode feedback with its discharge pathin shunt with the series combination of the network and the firstdevice, a connection for applying the anode voltage of the first deviceas a signal to a control electrode of the second device, a balancingimpedance element connecting the cathode of the first device to thecathode of the second device, said impedance element having a value suchas to reduce the variational admittance of the discharge path of thefirst device to zero, a by-pass tube, and means for switching thecurrent from said network to said by-pass tube under control ofswitching pulses.

5. The combination in series with a current source and a load in whichthe current is to be held constant, of two discharge devices and aresistor, each device being connected for cathode feedback, the resistorserving as the feedback resistance of the first device, and this deviceand the resistor together serving as the feedback resistance of thesecond device, and means for balancing residual current variations noteliminated by said devices, which comprises a third discharge deviceconnected for cathode feedback with its discharge path in shunt with theseries combination of the load and the first two devices, a connectionfor applying anode voltage of the second device as a Signal to the gridof the third device, and a balancing impedance element connecting thecathode of the first device to the cathode of the third device.

6. The series combination which comprises a current source, a load inwhich the current of the source is to be held substantially constantindependent of variations in the load, and a constant current path whichcomprises a discharge device having an anode, a cathode and a controlelectrode, and a resistance element having one terminal connected to thecathode of said device, the control electrode of said device beingmaintained at afixed potential with respect to the other end of saidresistance element, means comprising a switching tube for deprivingsaidload of said constant current, a cathode follower stage com-' prisinganother discharge device and a high cathode resistor, a connection fromthe anode of the first-named device to the grid of said follower, and aconnection from the cathode of said follower for Supplying anodepotential to said switching tube.

7. In combination with a current source and a network, apparatus forholding constant the current flowing from said source through saidnetwork, which comprisesa first discharge device connected for cathodefeedback with its discharge path in series with the network, a seconddischarge device connected for cathode feedback with its discharge pathin shunt with the series combination of the network and the firstdevice, a connection for applying the anode voltage of the first deviceas a signal to a control electrode of the second device, and a feedbackpath coupling the cathode of the first device to the cathode of thesecond device, said feedback path includ- I ing an impedance element ofa value such as to reduce the variational admittance of the dischargepath of the first device to zero.

8. In combination with a current source and a network, apparatus forholding constant the current fiowing from said source through saidnetwork which comprises a first triode and a resistor Rs connected forcathode feedback with the discharge path of the triode in series withthe network, a Second triode and a resistor R0 connected for cathodefeedback with the discharge path of said second triode in shunt with theseries combination of the network and the first triode,

a connection for applying the anode voltage of the first triodeas asignal to the grid of the second triode, and a balancing impedanceelement Rbccnnecting the cathode of the first triode to the cathode ofthe second triode, said impedance element being proportioned accordingto the formulae where ,ui is the amplification factor of the firsttriode, c is the-anode voltage of the firsttriode, and at is the-cathodevoltage of thesecond triode.

9. In combination with apparatus as defined in claim 8, a by-pass tube,and means for switching the current from said network to said by-passtube under control of switching pulses.

10. The combination in series with a current sourceand a load in whichthe current is to be held constant of two triodes and a resistor Rk,each triode being connected for cathode feedback, the resistor serving'as the feedback resistance-of the first triode and" this triode and theresistor together serving as the feedback resistance ofthe secondtriode, and means for balancing residual current variations noteliminated by said triodes, which comprises a third triode connected forcathode feedback with its dischargepath in shunt with the seriescombinationof the load and the first two triodes, a connection forapplying anode voltage of the second triode as a signal to the grid ofthe third triode, and a balancing impedance element Rb connecting thecathode of the first triode to the cathode of the third triode,saidi-mpedance element being proportioned according to the formulaewhere n is the amplification factor of the first triode,

2 is the voltage amplification factor of the second triode,

c is the anode voltage of the first triode, and

6c is the cathode voltage of the third triode.

11. The series combination which comprises a currentsource, a load inwhich the current of the source is to be held substantially constantindependent of load variations, and a constant current path whichcomprises a first discharge device having an anode, a cathode and asingle control. electrode, a second discharge device having an anode, acathode and a single control electrode, the anode of the first devicebeing connected to the cathode of the second device, a fixed potentialpoint, a resistor having one end connected to the cathode of the firstdevice and the other end connected to said fixed potential point, meanscomprising a potential source and connections of insubstantial impedancefor maintaining the control electrodes of both devices at potentialswhich are. fixed with respect to said fixed potential point, a pulsesource having two output terminals, a connection from one of. saidterminals to said fixed potential point, and. a connection from theother of said terminals to the cathode of one of said devices, wherebypulses of said source operate to switch the current in said path.

12. The series combination which comprises a current source, a load inwhich the current of the source is to be held substantially constantindependent of load variations, and a constant current path whichcomprises a first discharge device having an anode, a cathode and asingle control electrode, a second discharge device having an anode, acathode and a single control electrode, the anode of the first devicebeing connected to the cathode of the second device, a fixed potentialpoint, a resistor having one end connected to the cathode of the firstdevice and the other end connected to said fixed potential point, meanscomprising a potential source and connections of insubstantial impedancefor maintaining the control electrodes of both devices at potentialswhich are fixed with respect to said fixed potential point, means fordepriving said load of said constant current, which depriving meanscomprises a switching tube having a cathode connected to the cathode ofone of said devices, an anode con,- nected to said source and a controlelectrode, a pulse source having two output terminals, a connection fromone of said terminals to said fixed potential point, and a connectionfrom the other of said terminals to the. control electrode of saidswitching tube.

' LARNED A. MEACHAM.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,338,395 Bartelink Jan. 4, 19442,369,138 Cook Feb. 13, 1945 2,440,246 Cook Apr. 27, 1948 2,443,864MacAuley June 22, 1948 2,453,053 Wannamaker. Nov. 2, 1948

